The present invention relates to demodulation of radio signals modulated with a new, spectrally-efficient type of modulation while compensating for time dispersion caused by multipath propagation in the land-mobile radio environment.
The European digital cellular system known as GSM utilizes a modulation technique known as Gaussian Minimum Shift Keying (GMSK), in which successive binary bits are modulated alternately onto a cosine carrier and a sine carrier respectively while maintaining a constant signal amplitude. A related modulation called Offset-shaped Quadrature Phase Shift Keying ("Offset QPSK" or "OQPSK") provides a very similar signal but does not maintain a constant amplitude. Nevertheless, the similarity between GMSK and OQPSK waveforms is sufficient enough to make it possible for an OQPSK receiver to efficiently demodulate a GMSK signal and vice versa. Cellular telephones conforming to the GSM standard have been marketed and sold throughout the world by L.M. Ericsson since about 1990, and since about 1993 in the U.S.A. where the standard is known as PCS1900. L.M. Ericsson is the Swedish parent company of Ericsson Inc., the current assignee of this application. Ericsson GSM telephones utilize derotation of the received GMSK signal samples by successive multiples of 90 degrees. However, the information symbols that are derotated are symbols representing single binary bits that can take on levels of only +1 or -1, and not the multi-level signals disclosed in this application and its parent application. GSM telephones also employ channel estimation by correlating received signal samples with a known synchronization word ("syncword") and then using the channel estimates in a Viterbi processor to demodulate the received signal while compensating for intersymbol interference (ISI). Indeed, the Viterbi processor is a well known form of equalizer used to compensate for multipath propagation and other causes of ISI, and is described for example in U.S. Pat. Nos.: